1. Field of the Invention
The invention concerns power lasers pumped by laser diodes and, notably, a set of several power lasers pumped by laser diodes.
2. Description of the Prior Art
The emergence of laser diodes delivering high power and the adjustment of the transmission wavelength by a control of the composition of the epitaxiated layers or by using the properties of quantum well structures now makes it possible to envisage the making of coherent power sources using solid lasers with resonant pumping.
The use of this concept of pumping by laser diodes in order to make power lasers requires, conventionally, the use of transversally pumped slabs. Different approaches have been proposed. However, transversal pumping has some drawbacks:
the coupling between the linear arrays of laser diode and the slab is such that the heat exchanges at the slab are not easy; PA1 the heat dissipation at the laser diodes requires the use of bulky radiators, and the control of these pump lasers dictates the use of electrical conductors having a certain mechanical rigidity (to cope with strong currents). These drawbacks mean that the number of laser linear arrays that can be used for the transversal pumping is reduced to the minimum; PA1 the pumping efficiency cannot be optimized since the spatial distribution of the gain is the image of the distribution in intensity of the pump wave. The result of this is that the overlapping between the distribution of the mode in the cavity and that of the gain is close to that obtained by means of lamp or flash pumping (barring the spectral efficiency of pumping). However, we can see the advantage related to the use of a slab structure as compared with the slab with the property of having a higher distributed gain for an identical active medium since the optical path is obtained by a multiple total reflection (a zig-zag path). PA1 greater efficiency; PA1 high rate of repetition; PA1 reduction of stray thermal effects; PA1 etc. PA1 at least two laser diodes (1, 1') emitting a pump beam (Fp); PA1 at least two amplifier media (3, 3') each coupled to a laser diode (1, 1') and each having an input face receiving the pump beam emitted by the associated laser diode (1, 1') and retransmitting an output beam (Fs, Fs') by an output face (31); PA1 an optical coupling system (4, 40', 5) receiving the output beams (Fs, Fs') emitted by the amplifier media (3, 3') and retransmitting them as a common beam (Fc); PA1 an output mirror placed on the path of the common beam (Fc); PA1 each input face (30) being made so as to transmit the pump beam (Fp) received from a laser diode (1, 1') and so as to reflect the output beam (Fs, Fs') from the amplifier medium, the different input faces thus forming an optical cavity along with the output mirror.
One alternative consists in using sheets of optical fibers which, in being coupled to the pumping sources, make it possible, by off-settings, to dissociate the thermal problems at the pumping sources. However, this basic configuration has drawbacks related to fact that the coupling efficiency (input-output) in the fiber bundles is not 100% and that the distribution of the gain will always be less well adapted to the cavity mode as compared with that obtained by a longitudinal pumping.
The ultimate disturbing effects that could appear in the laser slab even in the presence of resonant pumping are also thermal.
In this case, the residual thermal effects are related to the quantum yield of the transfer which varies in the ratio (w.sub.1 /w.sub.p : laser transition frequency/absorption frequency i.e. pump frequency) and mean, for example, that if it is desired to obtain 1 mJ at 1.064 .mu.m from the transition .sup.4 F.sub.3/2 .sup.4 I.sub.9/2 of the Nd.sub.3+ ion, inserted in the YAG matrix, the pump energy at 0.808 .mu.m should be 1.3168 mJ. This pump wavelength characterizes the absorption band associated with the transition between the levels .sup.4 I.sub.11/1.sup.4 F.sub.5/2. In this case, the ratio w.sub.1 /w.sub.p is equal to 0.7594. This also means that 0.3168 mJ participates in the heating of the medium, this energy corresponding to the non-radiative transitions.
This observation is all the more important as the ratio w.sub.1 /w.sub.p is small. Thus, the use of pumping by diode lasers of the Tm3+ ion alone is expressed by a ratio w.sub.1 /w.sub.p =0.785/2.02=0.3886. This means that, to obtain 1 mJ at 2.02 .mu.m, it is necessary to use a pumping energy at 0.785 .mu.m of 1.6536 mJ which is accompanied by the dissipation of 0.6336 mJ characterizing the non-radiative transitions. Conversely, the use of a Tm.sup.3+ :Ho.sup.3+ codoping of the YAG makes it possible to obtain a ratio W.sub.1 /w.sub.p in taking account of effects, of the order of 0.74, of crossed relaxation oscillations and of transfer of the excited Tm.sup.3+ ion on Ho.sup.3.
These magnitudes show that the use of resonant pumping will have a limitation on the pumping efficiency laid down by the ratio w.sub.1 /w.sub.p. And the ultimate residual thermal effects that could develop in the laser crystal will be governed by this ratio.
The field covered by the present invention concerns solid-state power lasers pumped by laser diodes.
The use of laser diodes for the pumping of solid lasers has contributed to the revival of these coherent sources with the advantages proper to resonant optical pumping, namely:
The demonstrations have reported essentially in using longitudinal pumping. This limits the power levels that could be obtained. For, the length of the slab or, more generally, the efficiently excited volume, is restricted by the absorption of the medium at the pump wavelength. For example, for Nd:YAG, this length is equal to 5 mm. The obtaining of a higher power level then requires a transversal pumping of a slab or of a plate with a radius or thickness suited to absorbing the power of the pump.
Although the pump is resonant, a part of the power absorbed is, however, converted (through the excitation of the non-radiative transitions) into heat. Furthermore, the level of power required to pump and make a power laser dictates the use of a system for the removal of calories at the power laser diodes (it may be recalled that the efficiency of optical/electrical conversion in a laser diode is typically between 40% and 50%). There then arises the problem of designing a power laser head pumped by laser diodes, taking these problems into account.
The object of the present invention is based on the implementation of a new type of architecture of the pumping mode and cavity in order to make a power laser pumped by laser diodes reducing the residual thermal effects of the active medium to the minimum. It relies on the use of a longitudinal pumping of several laser slabs by laser diodes.